Embodiments of the present invention relate generally to active noise reduction systems. More specifically, embodiments of the present invention related to an optimized controller for use with active hearing protection devices.
Prolonged or high levels of sound exposure can induce hearing loss. A significant amount of prior research correlates overall A-weighted sound pressure levels with hearing loss metrics. Accordingly, the Occupational Safety and Health Administration guidelines state that by reducing the A-weighted sound pressure level (SPL) at a person's ear, safe exposure time limits may be increased and hearing health may be better preserved. The overall A-weighted level of a sound field is computed as a linear sound power sum over the audible frequency band, where the highest spectral levels will most influence the value of the overall sum. Therefore, hearing protection performance that targets the highest A-weighted levels first will be most effective. If all A-weighted octave band levels are the same, targeting most bands equally is needed to significantly impact the overall A-weighted SPL at the ear, and thus improve the hearing protection performance.
A multitude of hearing protection devices (HPD's) exist that are designed to limit the noise exposure at a person's ear. Both passive and active noise reduction devices are available on the commercial market including headsets, circumaural hearing protectors, and earplugs. Passive hearing protectors often gauge their effectiveness using a noise reduction rating (NRR) which predicts hearing protection performance in a flat broadband noise field. This is a broad ranging metric that indicates general protection in large number of different noise fields but it is not intended to represent optimized noise attenuation for any specific noise field or user.
The usual goal of commercial passive hearing protector designs is to achieve the highest NRR. However, this is not always a good indicator of the performance of the hearing protector compared to other protectors, or compared to the best design possible for a specific noise field that may be different from the pink noise used in the NRR calculation. Since hearing protectors using active noise control (ANC) are not typically evaluated even with the NRR, ANC designs are usually even less correlated with hearing protection performance than are passive designs. The prior art design criteria are primarily concerned with achieving high attenuation over a bandwidth determined by the open loop plant (i.e. the controller in series with the acoustic dynamics of the hearing protector) as well as the desire for a low complexity controller, rather than a consideration of A-weight noise field where the protector will be used.
Besides the lack of correlation between prior art ANC HPD's and reduction of A-weighted noise metrics, there are also deficiencies relative to the optimized performance of ANC HPD's for an arbitrary user. The primary reason for sub-optimal ANC HPD performance is related to the widely varying acoustic frequency response functions measured on an inter-person and even intra-person basis. The variations have resulted in ANC HPD's that emphasize robust closed-loop stability over optimal performance.
Typically, the compromise for ANC circumaural headsets is to rely on a large cup volume so that the acoustic mobility of the ear canal dynamics is not important relative to the acoustic mobility of the earcup's dynamics. Thus, the earcup design is selected to reduce inter-person variations. It is even possible to create intentional holes in the earcup volume to further improve the problem of plant variation from user to user. All of these approaches move away from ANC designs that yield optimal performance based on the actual acoustic frequency response for any particular user.
Prior art ANC earplug styles of HPD's have achieved robust performance through passive design of the acoustic plant to ensure that the earplug's acoustic frequency response (from speaker to microphone) is higher compliance than the ear canal compliance. This can only be achieved by relatively large volumes of space around the feedback microphone and therefore, must be accomplished at locations relatively far from the user's tympanic membrane. However, the distance between the feedback microphone and the tympanic membrane is directly correlated with the bandwidth of ANC that is effective at the tympanic membrane, where farther distances reduce the effective ANC bandwidth for the user. (See “Electronic Earplug For Monitoring And Reducing Wideband Noise At The Tympanic Membrane” U.S. application Ser. No. 10/440,619, which is incorporated herein by reference in its entirety for all purposes). Where variations in the open loop frequency response are designed away passively, as in using additional acoustic volume, optimal performance is sacrificed.
Attempts have been made to improve controller designs to account for additional variables. U.S. Pat. No. 6,665,410 issued to Parkins describes an active noise controller design approach that achieves the same performance for all individuals by altering the controller design to accommodate changes in the plant (the dynamics associated with the actuator, sensor, and acoustic dynamics in the occluded space). The controller is adjusted to produce a specified open loop response (controller in series with the plant). However, using a target open loop performance assures that some members of the user population will have plants that do not permit a realizable controller to achieve the target while other members will have plants that result in sub-optimum performance by application of the target. Ultimately, the optimal open loop shape varies from person to person and by designing the controller to achieve a fixed loop shape, almost all people will either not be able to attain the target design, or will not achieve optimal performance.
U.S. Pat. No. 5,600,729 issued to Darlington et. al. presents an adaptive feedback control technique that designs a controller in real time to minimize a noise impinging on a microphone. The configuration of the adaptive controller in the feedback loop can lead to instability for an arbitrarily small error in the plant identification required by the design process. Such a design is practically problematic since stability of the closed loop system during operation is not assured. In addition, Darlington does not specify a metric associated with hearing protection that is to be minimized.
Because the plant and passive control can change from person to person, a generalized controller design will actually be sub-optimal for all individuals. A fixed active controller design commonly applied to ANR hearing protection systems is a generic system that does not utilize any specific information about the user or noise field in which it operates. Such a static controller design that does not take into account any noise field characteristics, any passive control characteristics, any A-weighting or hearing protection weighting, or any plant dynamic characteristics that change from person to person, will result in hearing protection performance that is not the best achievable from that particular situation.
What is needed is an active noise controller that includes all of the necessary design variables to ensure the maximum available performance for every individual. Such a controller would achieve the best possible performance for each user by designing a unique controller to automatically maximize performance.